The present invention relates to an optical system support and positioning device for use in an apparatus for optically reading informations recorded on an optical disc information recorded on an optical disc or the like.
In an apparatus for optically reading information, information is recorded on a disc, such as a video disc or a digital audio disc, in the form of minute pits arranged in a spiral track on the disc surface. When a disc is reproduced, a beam of light is directed onto the track and the disc is rotated at a predetermined speed. The intensity of the reflected light or transmitted light is converted into an electrical signal, thereby reproducing the original information signal.
For the reproduction of such a disc, the light beam must be correctly focused on the recording surface of the disc. This requires positional control (focus servo) in a direction perpendicular to the recording surface of an optical system, that is, in the focusing direction. Further, the light beam should be controlled (tracking servo) radially of the disc, that is, in a tracking direction, in order to cause the light beam to always correctly follow the recording track. To this end, an optical system driving device for use in such an apparatus for optically reading information includes a support mechanism for movably supporting the optical system which controls the reading light beam and a positioning mechanism for controlling the position of the optical system in the focusing and tracking directions in response to focusing error and tracking error signals. Various types of such optical system support and positioning mechanisms have been heretofore proposed, one of which is illustrated in FIGS. 1 and 2.
As shown therein, an optical system 3 having an objective lens 1 and a barrel 2 is supported by a support element 4 so as to be movable in two directions, namely, a focusing direction Z parallel to the optical axis of the optical system 3 and tracking direction X perpendicular to the focusing direction. The support element 4 is composed of a pair of cantilevered flexible members 6 arranged parallel to one another with each having a first end fixed to a frame 5. An intermediate member 7 is arranged between the second, free ends of the flexible members. Support members 8 having one end connected to the intermediate member 7 extend in the direction of the fixed ends of the flexible members 6. The other ends of the support members 8 are connected to the optical system 3. The flexible members 6 and the support members 8 can flex only in the tracking direction X and the focusing direction Z. Reference numeral 9 indicates a rigid member. A coil 11, which is used to generate a force acting in the focusing direction Z, is wound on a bobbin 10 mounted downwardly of the barrel 2. A pair of coils 13, for producing movement in the tracking direction X, are wound on corresponding bobbins 12 which extend in the tracking direction X. Magnetic circuits 14 and 15, rigidly mounted on the frame 5, provide an electromagnetic driving force for the coils 11 and 13.
A disadvantage in this optical support and positioning device is that when the optical system is driven in the tracking direction, the optical system undergoes a desired parallel movement only when the frequency of the driving excitation current is low. When the frequency is above a certain critical point, due to a disturbance in the responsive frequency characteristics of the system, the optical system is caused to rotate.
It is, therefore, an object of the present invention to provide an optical system support and positioning device which causes no disturbance of responsive frequency characteristics and hence provides parallel movement at all frequencies of the driving excitation current.